1. Field of Art
The present invention relates to a flexible optical solar reflector or flexible second surface mirror which is assembled in an artificial satellite or other equipment containing therein precise instruments for controlling the intake and output of heat in or from the satellite to maintain the temperature in the satellite within a pre-set range. It relates also to a multilayer insulator having an excellent thermal insulation function.
It is requisite to maintain the temperature in an artificial satellite within a certain operable temperature range in order that the instruments contained therein are operated normally. For this purpose, a material serving as an optical solar reflector (hereinafter referred to as OSR) is applied on the exterior surface of the satellite. The known materials used for such a purpose include so-called OSR which suppress the intake of heat due to solar light and radiate heat, and multilayered insulator materials only for suppressing the intake of heat.
An OSR is comprised of a heat radiation layer made of a resin or glass; and a metal layer, normally made of silver, for reflecting solar light, and attached to the radiation surface of the satellite. The performance characteristics of the OSR may be defined by the solar absorptance (.alpha.s) indicating the degree of absorbing the solar light and by the thermal emittance (.epsilon.) indicating the degree of emitting the heat in the satellite. In order to maintain the temperature in the satellite within a pre-set range, .alpha.s should be low and .epsilon. should be high. It is also requisite that the changes in .alpha.s and .epsilon. be small.
2. Related Art Statement
Known OSR include flexible optical solar reflectors (hereinafter referred to as FOSR) each being composed of a thermoresistant polymer as a heat radiation layer, and rigid optical solar reflector (rigid OSR) including glass as the heat radiation layer.
The rigid OSR are excellent in durability in geosychronous orbit or aerospace environment so that the performance characteristics thereof are not deteriorated even after they have been used for a long time period. However, since they are in the shape of flat plate and are difficult to form with curved surfaces, they can only be used at limited locations on the satellite. Further disadvantages of the rigid OSR are that extreme care must be taken in handling thereof and that difficulties are encountered in assemblying them in the satellite, since they are made of thin glass plates.
In contrast thereto, the FOSR cause no serious problem in the assemblying operation and may be used at any locations without limitation. However, the conventional FOSR could not be used for a long period due to deterioration of the component resins under the influences of radioactive rays. The thermoresist resins used in the conventional FOSR were a transparent copolymer of tetrafluoroethylene and hexafluoropropylene, and transparent aromatic polyimides.
The known multilayer thermal insulator is composed of laminated polymer films each being coated with aluminium or silver by vacuum deposition. The .alpha.s and .epsilon. of the material used for forming the outermost layer of the multilayer thermal insulator affect the thermal insulation function of the specific multilayer thermal insulator. The materials used for forming the outermost layer of the known multilayer thermal insulator are an aromatic polyimide film (such as the polyimide film sold under the Trade Name of KAPTON) which is excellent in resistance to radioactive rays, and FEP Teflon which is excellent in .alpha.s.
An artificial satellite is held in a geosynchronous orbit or aerospace environment and thus subjected to high vacuum, extremely low temperature, irradiation of solar light and irradiation of radioactive rays including electron irradiation. Accordingly, materials for the OSR suffer various stresses. The FEP Teflon presently used as a material for FOSR is highly resistive to ultraviolet rays and heat, but it is known that the FEP Teflon apts to be deteriorated by electron irradiation. It has been pointed out that the main polymer chain of the FEP Teflon is cleaved by a chain reaction caused by electron irradiation, whereby the mechnical strength thereof approximates zero with disadvantageous increase in .alpha.s. A silverized FEP Teflon, which is predominantly used as a material for the FOSR is seriously deteriorated in its properties due to irradiation of radioactive rays, particularly electron irradiation, when it is held in a geosynchronous orbit or aerospace environment. For this reason, the FEP Teflon is not suited for use in an artificial satellite which is designed to have a long life. Under such circumstances, it is demanded to develop a novel material for the FOSR having a resistance to irradiation of radioactive rays.
Likewise, there is a demand for a material having a smaller .alpha.s value and an improved resistance to irradiation of radioactive rays so as to be adapted for use as a material for the outermost layer of the conventional multilayer thermal insulator.
On the other hand, an FOSR comprised of an aromatic polyimide film, which is excellent in resistance to radioactive rays, applied with a vacuum deposited aluminium coating and having a film thickness of 125 microns has only an unsatisfactory .alpha.s value of 0.5. A polyimide including therein an aromatic imide ring for improving the resistance to radioactive rays loses transparency, and thus such a material is hardly adapted for use as a convenient FOSR material. U.S. Pat. No. 3,917,643 discloses that a poly(phthalimide) having ether bonds and prepared through a reaction between a phthalic anhydride having an aromatic bis-ether bond and an organic diamine (this kind of poly(phthalimide)s being hereinafter referred to simply as poly(ether imide) may be used as an enamel for coating a wire. On the other hand, U.S. Pat. No. 4,024,101 discloses that a poly(ether imide) having aromatic bis-ether bonds and prepared through a reaction between bis(nitrophthalimide) and a diphenoxide of an alkali metal may be used as an injection molding material. It is known that this kind of poly(ether imide)s has a molecular structure containing a bond group for cleaving the conjugated system so that such a material has relatively high transparency. However, the organic diamine residue contained as a structural unit in the poly(ether imide) is apt to be oxidized. As a result of oxidation of the diamine or diamine residue, the transparency of the final product is lowered. In addition, if unreacted bis(nitrophthalimide) is present in the reaction product, the transparency of the final product is adversely affected. T. E. Attwood et al. have reported on poly(arylether ketone), reference should be made to Polymer, Vol. 22, August 1981, Pages 1096 to 1103.) For example, poly[(4-phenoxy-4'-l -carbonyl)diphenyl ether] can be synthesized by polycondensation of bis-4-fluorophenyl ketone and a potassium salt of hydroquinone.